Anaxagoras is famous for introducing the cosmological concept of Nous (mind), as an ordering force. He regarded material substance as an infinite multitude of imperishable primary elements, referring all generation and disappearance to mixture and separation respectively.

Cosmological theory

All things have existed from the beginning. But originally they existed in infinitesimally small fragments of themselves, endless in number and inextricably combined. All things existed in this mass, but in a confused and indistinguishable form. There were the seeds (spermata) or miniatures of wheat and flesh and gold in the primitive mixture; but these parts, of like nature with their wholes (the homoiomereiai of Aristotle), had to be eliminated from the complex mass before they could receive a definite name and character.

Mind arranged the segregation of like from unlike; panta chremata en omou eita nous elthon auta diekosmese. Nous knew all things and directed the mixing together and separation of things from each other. However, this separation is incomplete – there is still a share of everything contained in each thing. Only Nous remains unmixed. While Nous sets the revolution in order, it has a special relationship to living things. Specifically, Anaxagoras believes that Nous is the ruler of living things. He uses the term κράτος, or kratei to indicate that Nous rules living things.

Mind causes motion. It rotated the primitive mixture, starting in one corner or point, and gradually extended until it gave distinctness and reality to the aggregates of like parts, working something like a centrifuge, and eventually creating the known cosmos. But even after it had done its best, the original intermixture of things was not wholly overcome. No one thing in the world is ever abruptly separated, as by the blow of an axe, from the rest of things.

It is noteworthy that Socrates (Plato, Phaedo, 98b) accuses Anaxagoras of failing to differentiate between nous and psyche, while Aristotle (Metaphysics, Book I) objects that his nous is merely a deus ex machina to which he refuses to attribute design and knowledge.

Anaxagoras proceeded to give some account of the stages in the process from original chaos to present arrangements. The division into cold mist and warm ether first broke the spell of confusion. With increasing cold, the former gave rise to water, earth and stones. The seeds of life which continued floating in the air were carried down with the rains and produced vegetation. Animals, including man, sprang from the warm and moist clay. If these things be so, then the evidence of the senses must be held in slight esteem. We seem to see things coming into being and passing from it; but reflection tells us that decease and growth only mean a new aggregation (synkrisis) and disruption (diakrisis). Thus, Anaxagoras distrusted the senses, and gave the preference to the conclusions of reflection. Thus, he maintained that there must be blackness as well as whiteness in snow; how, otherwise, could it be turned into dark water?

Anaxagoras marked a turning-point in the history of philosophy.
With him, speculation passes from the colonies of Greece to settle at Athens. By the theory of minute constituents of things, and his emphasis on mechanical processes in the formation of order, he paved the way for the atomic theory.

Literary references

Anaxagoras appears as a character in The Ionia Sanction, by Gary Corby.

Anaxagoras is referred to and admired by Cyrus Spitama, the hero and narrator of Creation, by Gore Vidal. The book contains this passage, explaining how Anaxagoras became influential:

[According to Anaxagoras] One of the largest things is a hot stone that we call the sun. When Anaxagoras was very young, he predicted that sooner or later a piece of the sun would break off and fall to earth. Twenty years ago, he was proved right. The whole world saw a fragment of the sun fall in a fiery arc through the sky, landing near Aegospotami in Thrace. When the fiery fragment cooled, it proved to be nothing more than a chunk of brown rock. Overnight Anaxagoras was famous. Today his book is read everywhere. You can buy a secondhand copy in the Agora for a drachma.

William H. Gass begins his novel, The Tunnel (1995), with a quote from Anaxagoras: "The descent to hell is the same from every place."